1. Field of the Invention
This invention relates generally to the application of electrical energy in the heating of subsurface hydrocarbonaceous formations including tar sands and other viscous oil bearing formations. More particularly it is concerned with a method and apparatus for accomplishing this purpose utilizing an electrode well forming part of an electric circuit extending through a formation to be heated.
2. Description of the Prior Art
Known techniques and apparatus for electrical heating of a formation typically include sinking a well or wells into a oil bearing information or into immediately adjacent layers above and below such formation. A formation-contacting electrode may be formed as part of an alternating current circuit extending through the wellbore from the surface, the circuit being completed through the formation. The electrode is typically connected to a section of conductive casing which is in turn connected to an electrical cable extending downward from a power source at the surface. When power is applied an expanding electric field is created about each such electrode the current density within such field being greatest at the electrode itself. The smaller the surface area of the electrode the greater the current density for any given power and the greater the resultant heating. If current density becomes sufficiently high at the electrode, local heating may cause formation fluids to boil off, thereby interrupting current flow and the entire heating process. In order to overcome this problem it is advantageous to enlarge the contact area between the well electrode or electrodes and the adjacent formation. Greater power may then be applied in the heating process without reaching undesirable current densities. One known technique for this purpose is to create a hydraulic fracture filled with highly conductive proppant particles as described, for example in U.S. Pat. No. 3,547,193 to Gill or U.S. Pat. No. 3,862,662 to Kern. These conductive particles when interconnected with a source of potential through the well casing constitute an electrode of considerable contact area with the formation. Utilizing a hydraulic fracture zone as an electrode lowers current density in the immediate vicinity of the electrode well, thus minimizing local heating. Nonetheless, making the fracture conductive necessarily causes some heating to occur within the fracture, thus increasing its fluid mobility and therefore enhancing production when the electrode well is used for that purpose.
Hydraulic fractures created for the purpose outlined above typically involve perforation of the conductive section of casing, such perforations being continued through the cement liner into the formation itself. In order for the conductive proppant particles, such as steel shot, for example, to function as a large area electrode a good conductive path must be established and maintained between the proppant and the casing. In creating such a fracture, after the proppant particles have been introduced, the fracture fluid is "back produced". This should cause these particles to flow back into the perforations, but there is no reliable way to establish the extent to which such perforations are actually filled in this manner. All current flow into or out of the well will be narrowly confined to the surface area of the perforations within the conductive casing itself. If the proppant fails to work down into these perforations the required current paths into the proppant are never established and the process becomes non-operative. Even if the requisite contact is made, there is the drawback that the well can not thereafter be used for production through the fracture, since a tightly plugged perforation will restrict fluid flow.
It is therefore an object of this invention to provide an improved method and apparatus for electrically heating a formation.
It is a more particular object of this invention to provide a method and apparatus for forming an electrode well of greater efficiency utilizing a hydraulic fracture.
Other and further objects and advantages of this invention will become apparent from a consideration of the detailed description and drawings to follow taken in conjunction with the appended Claims.